An arc sensor used for searching a welding line of an arc welding robot and otherwise, is mounted on a welding torch of the robot, and because of this, the arc sensor experiences internal structure damage or accumulates stains on the lens as a result of spatterings and lampblack generated during welding. Therefore the arc sensor is provided with a replaceable guard window to guard the internal structure from spatterings and lampblack.
However, this guard window is sometimes burned black by the spatterings and accumulates stains as a result of lampblack and etc. in proportion to the number of welding operations, so that an incident beam and a reflected beam of a laser beam cannot easily penetrate the window. When penetration of the incident beam and reflected beam through the window becomes difficult, the detecting capability of the arc sensor is reduced and the detecting accuracy of the same is also reduced. Therefore, the guard window is usually replaced at determined intervals or as required, or the stains on the window are wiped clean after discerning the accumulated stains on the window with the naked eye.
However, if the guard window is replaced at determined intervals, stains accumulate and replacement thereof is quite often too late, in the case of a large number of welding operations, and as a result, the detecting capability of the arc sensor is reduced. On the other hand, replacement thereof can be too early, and a window is unnecessarily replaced in the case of a small number of welding operations during the determined interval.
Replacement of the guard window as required is indiscriminate and unreliable depending on individual operators thereof, for instance, by discerning accumulated stains with the naked eye, thereby posing the same problem as the case when the windows are replaced at determined intervals.